The present invention relates to a leaf spring, in particular for automotive suspensions, and to a relative fabrication method.
Automotive leaf springs are known which are made entirely of composite material, i.e. polymer resin reinforced with appropriate, e.g. glass, fibers, and which, like all items made of composite material, have superior mechanical characteristics and are lightweight as compared with similar conventional, e.g. metal, components.
A further reduction in the weight of leaf springs made of composite material could be achieved by varying the cross section: as is known, leaf springs, in actual use, are not stressed equally along the whole of their length, and only require given particularly thick sections (typically the connecting sections subjected to most stress).
One known method of producing leaf springs of varying thickness, described in Japanese Patent n. 56/139921, provides for winding about a removable frame a continuous strip of fibers preimpregnated with polymer resin, so as to form a number of superimposed layers of preimpregnated fibers: by varying the length of successive windings, it is possible to obtain a manufactured article of maximum thickness at the center, decreasing in thickness towards the ends, and which is then placed inside a mold to set the polymer resin and so obtain the finished article. Though enabling, unlike other known methods, the use of whole fibers along the whole length of the leaf spring (the best working condition for the fibers), the above method nevertheless involves several drawbacks. In particular, leaf springs are formed containing fibers throughout the full thickness, and in particular also along the neutral axis where, as is known, the presence of fibers, as opposed to improving performance of the leaf spring, only serves to increase weight and reduce resistance to shearing stress (which is greatest at the neutral axis, and which, as is known, is not adequately withstood by the fibers). Moreover, any connecting holes (indispensable for connecting the leaf spring to other mechanical vehicle members, unless alternative complex high-cost fastening systems are resorted to) must be formed through the composite material, thus locally interrupting (and so impairing performance of) the fibers, locally increasing stress, and, in other words, significantly reducing resistance precisely at the most highly stressed connecting points.
It is an object of the present invention to eliminate the aforementioned drawbacks of leaf springs produced using known methods. In particular, it is an object of the invention to provide a leaf spring of superior mechanical characteristics and extremely light weight; it is a further object of the invention to provide a straightforward, low-cost method of fabricating such a leaf spring.
According to the present invention, there is provided a leaf spring, in particular for automotive suspensions, characterized by comprising a central core made of a first polymer material of predetermined mechanical characteristics and having no reinforcing fibers; and at least one cover for covering said central core and made of a composite material in turn comprising a polymer matrix incorporating a number of one-way reinforcing fibers; said central core being of a shape corresponding to the conformation of said leaf spring when undeformed, and being defined by two opposite longitudinal, elastically bendable work surfaces, and by two lateral sides connecting said work surfaces; said at least one cover for covering said central core being located on at least one of said work surfaces of said central core.
The leaf spring according to the invention preferably comprises a predetermined number of transverse holes formed through said central core of polymer material and by which to fasten said leaf spring to respective mechanical members; said transverse holes being formed through said sides of said central core so as not to intersect said at least one cover of composite material.
According to the present invention, there is also provided a method of fabricating a leaf spring of varying cross section, in particular for automotive suspensions, the method being characterized by comprising the steps of:
producing, from a first polymer material of predetermined mechanical characteristics and having no reinforcing fibers, a form elongated longitudinally and defined by two opposite, facing, longitudinal work surfaces, and by two lateral sides connecting the longitudinal said work surfaces; said form having a longitudinal contour substantially coincident with the required contour of said leaf spring when undeformed, and having a longitudinal axis substantially coincident with a neutral axis of said leaf spring;
impregnating a number of one-way, longitudinally aligned reinforcing fibers with a polymer resin in the fluid state to obtain corresponding preimpregnated one-way reinforcing fibers;
placing at least one respective layer of said preimpregnated one-way reinforcing fibers onto each of the two said work surfaces of said form so as to follow the longitudinal contour of the form; said respective layers of said preimpregnated one-way reinforcing fibers being so placed that the reinforcing fibers are substantially parallel to said longitudinal axis of said form;
bringing about a polymerization and/or cross-linking reaction of said polymer resin so that said respective layers of said preimpregnated one-way reinforcing fibers on the two said work surfaces of said form define respective laminates of composite material closely connected to, and reproducing the longitudinal contour of, said form; said form of said first polymer material defining a central core of said leaf spring, of which said respective laminates of composite material define a cover.
This therefore provides for obtaining cheaply and easily a leaf spring of superior mechanical characteristics and extremely light weight : the central core of the leaf spring, located at the neutral axis of the spring, is made easily, even of varying thickness and extremely lightweight from polymer material with no reinforcing fibers; precisely by having no fibers, the central core is of excellent resistance to shearing stress (severest at the neutral axis), especially if made of epoxy resin; and the absence of fibers in this part of the leaf spring not only provides for reducing the overall weight of the spring, but also for improving shear strength and fatigue performance with no negative effect on the mechanical characteristics of the spring. Moreover, while providing for leaf springs of both constant and varying cross section, the method according to the invention is particularly suitable in the latter case: to obtain a leaf spring of varying thickness, in fact, the central core of polymer materialxe2x80x94easily shaped using straightforward, low-cost techniquesxe2x80x94need simply be of varying thickness, whereas the cover laminates of composite material may advantageously be made of constant thickness, i.e. with no resort to complex processing of the composite material. Clearly, however, the cover laminates of composite material may also be of varying thickness, as would be the case, for example, if the laminates were formed by superimposing successive layers of different lengths.
According to a preferred embodiment of the invention, use is advantageously made of continuous reinforcing fibers, which are only interrupted at the neutral axis of the leaf spring, and which are therefore fully functional. Also for preventing interruption and functional impairment of the reinforcing fibers, the fastening holes of the leaf spring are formed in the central core of resin, and not through the cover layers of composite material.
An epoxy resin is preferably used both for the central core and as a matrix of the composite material of the cover layers: the epoxy resin defining the matrix of the composite material is advantageously polymerized in two stages according to a known technique.